A large variety of cheeses are produced by clotting raw or pasteurized milk using acid, rennin or both. In the past, the rennin used is often obtained from rennet. This rennin is in nature an enzymatic protein used for coagulating milk casein in cheese making. It also acts as a cheese ripening material in the latter stages of cheese making through its proteolytic activity. Rennet has in the past been obtained from butchered milk-fed calves' stomachs processed in accordance with known techniques.
More recently, rennin or prorennin has been formed by recombinant DNA techniques, see for example European patent publication No. 82 110 124.5 filed Jan. 8, 1982 Publication No. 0057,350,A3 dated Aug. 11, 1982 Bulletin 82/32.
In the past, it has been suggested that where rennin or prorennin is produced from cellular materials, purification should be carried out to obtain the purified enzymatic material capable of activation to have milk clotting activity.